(a) Field of the Invention
The present invention relates to a radio frequency identification tag antenna.
The present invention was supported by the IT R&D program of is MIC/IITA [2006-S-023-02, Development of Advanced RFID System Technology].
(b) Description of the Related Art
A radio frequency identification (RFID) tag is used in various fields such as distribution and material handling industries, together with an RFID reader.
When an object to which the RFID tag is attached accesses a read zone of the RFID reader, the RFID reader generates an interrogation signal by modulating an RF signal having a specific frequency, the RFID reader transmits the interrogation signal to the RFID tag, and the RFID tag responds to the interrogation received from the RFID reader.
That is, the RFID reader transmits an interrogation signal to the RFID tag by modulating a continuous electromagnetic wave having a specific frequency. The RFID tag transmits back the electromagnetic wave transmitted from the RFID reader after performing back-scattering modulation in order to transmit its own information stored in the RFID tag's internal memory. The back-scattering modulation is a method for transmitting tag information by modulating the amplitude and/or the phase of a scattered electromagnetic wave when the RFID tag transmits the electromagnetic wave that is initially transmitted from the RFID reader back to the RFID reader by scattering the electromagnetic wave.
A typical passive RFID tag includes an RFID tag chip and an RFID tag antenna. At this time, the RFID tag antenna further includes a feeding line with a loop shape, which supplies power to the RFID tag chip. Since a passive RFID tag does not include a separate operation power source, it rectifies the electromagnetic wave transmitted from the RFID reader and uses the rectified electromagnetic wave as its own power source to acquire operation power. That is, the RFID tag chip modulates the amplitude and/or the phase of the electromagnetic waves received from the RFID reader in accordance with identification information, and the RFID tag antenna scatters the electromagnetic waves modulated by the RFID tag chip.
The intensity of the electromagnetic wave transmitted from the RFID reader should be larger than a specific threshold value for normal operation. However, since the transmission power of the reader is limited by local regulations of each country, it is not possible to unconditionally raise the level of transmission power. Therefore, the RFID tag should efficiently receive the electromagnetic wave transmitted from the RFID reader to extend the read zone without raising the transmission power level of the reader.
A method for raising the receiving efficiency of the RFID tag is to perform complex conjugate matching of an antenna and a radio frequency (RF) front-end of the RFID tag chip so as to maximize the intensity of the signal received by the RFID tag. In this way, the RFID tag can receive the electromagnetic waves with maximum intensity from the RFID reader.
In designing the RFIF tags, since each antenna should be design to impedance-match with impedance of a corresponding RFID tag chip, a large design time is required.
In addition, the typical RFID tag has a drawback of difficulty in matching with a reactance component of impedance between the RFID antenna and the RFID chip due to the large feed line with the loop shape.
Accordingly, there are difficulties in miniaturizing the RFID tag antenna and reducing cost.